In recent years, short-range optical communication increases at a rate of 30% annually, and this rate will further accelerate along with continuous development of the mobile Internet. Exactly because of the great potential of the short-range optical communication, more attention is diverted from long-range optical communication to the short-range optical communication. In short-range application, people pay more attention to an Intensity Modulation/Direct Detection (IM/DD) technology. As a rate of a short-range system continuously increases, it is an inevitable trend that the rate increases from 25 Gbps to 100 Gbps for IM/DD. However, a currently available component bandwidth is only 25 GHz. Therefore, spectral efficiency needs to be improved, so that a transmission rate can reach 100 Gb/s.
As shown in FIG. 1, FIG. 1 a left and right sideband modulation method commonly used at present. On a first channel, a digital signal A is modulated into a single sideband digital signal a, and a carrier frequency is fm.a=A*cos(2*π*fm*t)+{circumflex over (A)}*sin(2*π*fm*t)
Likewise, on a second channel, a digital signal B is modulated into a single sideband digital signal b, and a carrier frequency is fm.b=B*cos(2*π*fm*t)+{circumflex over (B)}*sin(2*π*fm*t)
The digital signals a and b are modulated into an optical carrier by means of phase shifting and digital-to-analog conversion, to obtain left and right sideband digital signals in an optical domain, as shown in FIG. 2. A left sideband carries information about a, and a right sideband carries information about b. It can be learned that, in this solution, a spectrum is seriously wasted, and spectral efficiency is low.